Negative electrode active materials such as silicon are expected to have higher capacities than carbon materials such as graphite. However, such negative electrode active materials undergo significant changes in volume during charge and discharge. In association with these significant changes in volume, the current-collecting abilities of negative electrodes are reduced, and thus the cycle characteristics are deteriorated.
In order to improve the current-collecting ability, a proposal has been made to mechanically coat the surface of silicon oxide particles with a carbon material (see Patent Document 1), and another proposal has been made to form carbon fibers on the surface of an active material core containing a metal or semimetal capable of forming a lithium alloy, for example, by a CVD method (see Patent Document 2).
Further, in order to inhibit the side reaction between the negative electrode active material and the non-aqueous electrolyte, a proposal has been made to add LiBF4 to the non-aqueous electrolyte (see Patent Document 3), and another proposal has been made to add LiB(CxH2(x-2)O4)(CyH2(y-2)O4) and LiBF4 to the non-aqueous electrolyte (see Patent Document 4).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-373653    Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-349056    Patent Document 3: Japanese Laid-Open Patent Publication No. 2003-45488    Patent Document 4: Japanese Laid-Open Patent Publication No. 2005-79057